The basic structure of a high electron mobility transistor, HEMT, has a source and drain structure with a heterojunction formed by modulation-doped channel layer and donor-supply layer, typically consisting of an undoped GaAs channel layer and a highly doped n-type AlGaAs donor-supply layer. Using a single doped AlGaAs and undoped GaAs heterojunction to achieve field effect control of electron accumulation at the interface. The electrons, two dimensional electron gas, 2-DEG, generated in the thin junction layer, confined by quantum effects to a thin sheet, are free to move along this thin layer without hindrance and interference of doped ionized impurities, resulting high electron mobility allowing fast response times and low noise operation. HEMT is commonly used in voltage control and regulation devices, by varying the gate voltage, Vg, to change the depth of hetero-junction potential well, to vary the sheet charge density of 2-DEG achieving the regulation of the working current.
For GaAs based HEMT, normally the heavily doped n-AlxGal-xAs control layer is depleted. For depletion mode device, the n-AlxGal-xAs is thicker and heavily doped, 2-DEG exist even at Vg=0. Otherwise when the device is enhancement-mode, at Vg=0, Schottky depletion layer extended to GaAs layer; Hence, for HEMT, the main influencing factor is the doping density and the especially the thickness of wide band gap semiconductor layer. The surface density of 2-DEG, Ns, in HEMT, is mainly influenced by the sub-band of potential well of the heterojunction (i=0 and 1). 2-DEG surface charge density is Vg regulated.